1. Field of the Invention
The present invention relates to copiers, printers, facsimile devices, and other image-forming devices for forming images with electrophotography, and a charged device, cleaning device, process cartridge, and toner that are adopted therein.
2. Description of the Background Art
In this type of electrophotographic image-forming device, a charge with a prescribed polarity is imparted and retained by discharging electricity on the surface of a photoreceptor or other image carrier, the charged photoreceptor surface is exposed to form an electrostatic latent image, toner charged with the same polarity as the charged polarity is fed to the electrostatic latent image, and a toner image is formed. The toner image formed on the surface of the image carrier is transferred to recording paper or another medium, and heat and pressure are applied to fix the image to the recording paper or other medium. Also, since there is residual toner that is not transferred on the surface of the image carrier after the toner image has been transferred, the surface is cleaned with a cleaning blade, cleaning brush, or another cleaning device prior to entering the next charging step.
A conventional configuration of a non-contact method that uses corona discharge is disclosed in Japanese Laid-open Patent Application No. 8-106203 (paragraphs “0002” and “0003”), for example, as a charged device adopted in such an image-forming device. There are drawbacks in a charged device that uses the non-contact method in that ozone, nitrogen oxide, and other corona products associated with corona discharge cause adverse environmental effects, and the charging characteristics can deteriorate when nitric compounds (ammonium nitrate) and other discharge products based on nitrogen oxide are deposited.
In lieu of such a non-contact charged device, also known are charged devices that use a contact method whereby the charged roller is brought into contact with the surface of the image carrier to impart a charge, and charged devices that use a close proximity method whereby the charged roller is brought into close proximity to the surface of image carrier while maintaining a small gap of several microns to several tens of microns to impart a charge.
Normally, a charging bias voltage is applied to the charged roller of charged devices that use the contact or close proximity methods. An AC voltage that has a DC voltage and an interpeak voltage that is no less than double the discharge starting voltage of the DC voltage is superposed and applied as such a charging bias voltage, the electric potential of the latent image carrier is converged to the value of the applied DC voltage by the application of the AC voltage, and a uniform charge can be imparted to the surface of the latent image carrier as a result.
Described next are various problems that need to be solved in charged devices based on the use of the above-described contact and close proximity methods.
[1] First Problem
In charged devices using the above-described close proximity method, a small gap is formed between the surface of the image carrier and the surface of the charged roller, and disclosed in Japanese Laid-open Patent Application No. 2003-66693 (paragraphs “0010” and “0011”), for example, is a technique in which the surface of the latent image carrier is placed in a state of constant etching since the surface of the image carrier is charged by a pulse discharge generated between the small gap. When the surface of the image carrier is cut by the etching phenomenon produced by the charged roller, a film-thinning phenomenon is created whereby the film thickness of the photosensitive layer of the surface thereof is reduced, and it is possible that the charging characteristics may deteriorate and the image quality may decline as these phenomena occur.
For example, Japanese Laid-open Patent Application No. 2003-66693 discloses a method aimed at reducing the film-thinning phenomenon of such a latent carrier, whereby a DC voltage is applied exclusively to the charged roller that makes contact, and, for example, Japanese Laid-open Patent Application No. 2003-91143 (paragraph “0010”) discloses a method in which a lubricant is applied to the surface of the latent image carrier.
In the former method, the surface of the image carrier is charged solely by DC voltage and the amount of current flowing to the image carrier is therefore considerably reduced because AC voltage is not applied thereto. In other words, the pulse discharge to the image carrier is reduced, and, as a result, the etching effect on the surface of the image carrier is reduced and the film-thinning phenomenon of the image carrier controlled. Used in the latter method is a solid lubricant application device that is provided exclusively to the charged device independent from the cleaning device of the image carrier, and the surface can be protected by forming a lubricating layer on the surface through the application of zinc stearate or another lubricant, for example, to the surface of the image carrier to increase the abrasion resistance.
There is an additional problem in that the charged roller becomes soiled, which is another reason that the image quality may decline due to the deterioration of the charging characteristics of the image carrier when a charged roller is used. The charged roller is disposed facing the surface of the image carrier which has undergone the cleaning step, but is also disposed in a manner that tends to allow toner, paper dust, and other unwanted matter left behind in the cleaning step to be deposited on the surface of the image carrier. For this reason, the surface friction changes when partial soiling occurs on the charged roller, and, as a result, a homogeneous charge cannot be imparted to the image carrier. In view of the above, for example, Japanese Laid-open Patent Application Nos. 2003-29430 (paragraph “0012”), 7-11425 (paragraph “0023”), and 2002-108069 (paragraph “0026”) disclosed conventional methods in which a cleaning blade, a cleaning member in the form of a pad, or a brush is provided to the charged roller, and, for example, Japanese Laid-open Patent Application No. 6-149012 (claim 1) discloses the provision of a cleaning roller equipped with a cleaning blade.
Considering the soiling problems of such a charged roller, the film thinning of the latent image carrier can be reduced in the method for applying DC voltage exclusively to the charged roller as described above, but partial soiling tends to occur in that toner, paper dust, and other unwanted matter is deposited on the surface of the charged roller, and, as a result, the electrical resistance on the surface thereof may become nonuniform. When the uniform charge on the image carrier is degraded, a slight variation in the friction causes image smudges or nonuniformity in the image because AC voltage cannot be applied. Also, in the method in which lubricating oil is applied to the surface of the image carrier described above, the cleaning characteristics in the environmental variations of the image carrier are stabilized by reducing the friction coefficient with the lubricant, but a portion of the toner or paper dust deposited on the surface of the image carrier with reduced friction more easily slips away from the cleaning position, and changing the friction of the surface when toner, paper dust, and other unwanted matter are deposited on the charged roller has the same result as described above with respect to soiling the charged roller.
The lifespan of the image carrier can be extended by inhibiting the film-thinning phenomenon on the surface of the image carrier in this manner, but in recent years, configurations in which the image carrier, the charged device therein, the developing device, and the cleaning device are housed together in the process cartridge are becoming more widespread because of the improved maintenance characteristics, and it is important from the aspect of reducing running costs to make the lifespan of all the housed devices the same, rather than to extend the lifespan of only a portion of the housed devices.
In a configuration for allowing the soiling of the charged roller to be prevented, a configuration for recovering the foreign matter removed from the charged device is required. In the particular case that a blade is used, there is a requirement to control the setting and other parameters of a recovery timing and a recovery unit that is separate from the blade since the blade itself cannot hold the foreign matter. For this reason, the charged device configuration becomes more complicated, resulting in higher costs.
When a cleaning member in the form of a pad or a sponge is used, the captured foreign matter must be retained, but it is difficult to retain the foreign matter simply by bringing the cleaning member into contact with the charged roller. For this reason, when a cleaning structure is provided to the charged roller in either case, a mechanism for capturing foreign matter and a recovery mechanism is required in addition to the charged roller and cleaning structure, and higher costs due to the greater size and complexity of the device are unavoidable. Such drawbacks are not limited to charged devices that simply use a charged roller, but also apply to transfer devices and other devices that may involve contact with the latent image carrier.
[2] Second Problem
While the demand for higher image quality and smaller configurations has increased in recent years, toners with smaller, spherical-shaped particles have come to be used in the development step. There are attempts to densely deposit toner in the electrostatic latent image through the use of such toners. However, the above-described toners with smaller, spherical-shaped particles tend to slide on the cleaning blade in the cleaning step, and cleaning tends to be inadequate. In other words, residual toner on the surface of the image carrier adheres to the charged roller without being cleaned away, and the surface of the photoreceptor cannot be uniformly charged. Therefore, to prevent such a situation, the surface of the charged roller must be cleaned.
For example, Japanese Laid-open Patent Application No. 5-297690 discloses polyurethane foam, polyethylene foam, or another sponge material that serves as a cleaning member of such a charged roller, and, for example, Japanese Laid-open Patent Application No. 2002-221883 discloses a brush roller. These cleaning members remove toner and other deposits by making contact and rubbing against the surface of the charged roller. In the case of sponge material, the deposits are held in the cells contained therein, and in the case of a brush, the deposits are held between the fibers of the brush.
However, there is a limit to the amount of deposits that can be held in the cleaning member, and there is an unresolved issue with regard to maintaining the cleaning characteristics of the cleaning member over a long period of time. In a process cartridge configured with a charged roller, for example, the performance of the charged roller, and consequently the cleaning function of the surface of the charged roller, must be in agreement with the lifespan of the other configurational components, and the above-described cleaning members are inadequate for such an object.
In view of the above, the use of a cleaning member composed of a melamine resin foam having a three-dimensional reticulated structure is disclosed in Japanese Laid-open Patent Application No. 2003-66807, for example, as a replacement to the above-described cleaning members, whereby the performance of the charged roller can be maintained over a long period of time. Such a cleaning member does not allow unwanted matter to clog a single cell as does a conventional sponge material, and the cleaning characteristics of the surface of the charged roller can therefore by maintained over a long period of time.
However, when a configuration is adopted whereby the cleaning member is brought into contact with the charged roller by its own weight, for example, and when contact between the charged roller and cleaning member continues for a long period of time in a state in which the charged device is stopped, a problem is encountered in the sense that a contact mark may be left on the surface of the charged roller, and charging thereafter may not be uniform, leading to the generation of abnormal images. This phenomenon particularly tends to occur when the contact time is extensive at high temperatures.
[3] Third Problem
In order to remove foreign matter deposited on the surface of a charged roller such as that described above, there is disclosed in Japanese Laid-open Patent Application No. 14-169327 a configuration that provides a charged cleaning member that rubs against the surface of the charged roller to remove toner, paper dust, and other unwanted matter on the surface thereof. In other words, the above publication discloses a charged cleaning roller that removes toner, paper dust, and other unwanted matter from the surface of the charged roller by rubbing against the surface in conjunction with the rotation of the charged roller. Such a charged cleaning roller is advantageous in that the cleaning durability thereof is on a par with that of a cleaning pad or another fixed-type charged cleaning member. Furthermore in the above-noted publication, the configuration has a layout arrangement in which the charged roller is disposed above the photoreceptor in the vertical direction, and the charged cleaning roller makes contact with the charged roller by its own weight and is configured to rotate in conjunction with the charged roller. Hereinafter, this arrangement is referred to as an “upper-side arrangement.”
However, due to the constraints of the layout arrangement of the entire device, contact cannot be made using the deadweight of the charged cleaning roller when the charged cleaning roller makes contact with the surface of the charged roller at a position lower than the virtual horizontal plane containing the center of rotation of the charged roller. In view of the above, pressure is applied to the shaft of the charged cleaning roller brought into contact with the charged roller so as to rotate in accompaniment therewith. In the layout in FIG. 1, the transfer unit is disposed above the photoreceptor in the perpendicular direction, the charged roller is disposed below, and the charged cleaning roller is disposed below the charged roller in the perpendicular direction. Hereinafter, this arrangement is referred to as a “lower-side arrangement.” The charged roller and charged cleaning roller incur the following problems in such a layout.
In other words, extraneous stress is unavoidably placed on the charged roller and the charged cleaning roller in comparison with the upper-side arrangement in which the deadweight was used, because the shaft of the charged cleaning roller is pressed with a constant force to make contact with the charged roller and is caused to rotate in accompaniment therewith. When this configuration is used over a long period of time, the charged roller or the charged cleaning roller becomes soiled, linked rotation does not proceed smoothly, and the cleaning characteristics may worsen. A lubricant is applied to the surface of the photoreceptor in order to protect the surface of the photoreceptor from hazards produced by charging, for example, but when the lubricant is deposited on the surface of the charged roller over time, the coefficient of friction of the contact portion decreases considerably, and linked rotation does not proceed smoothly.
In view of the above, when the contact pressure is set high in order to maintain frictional force, the slide load of the shaft increases and the rotation of the charged cleaning roller is inhibited. Also, the lubricant and additives and the like in the toner are particularly easily deposited on the charged roller, and when the pressure is increased the deposits rub against the charged roller creating a film, and the film produces nonuniform resistance on the surface. For this reason, abnormal images are easily generated due to a nonuniform charge, insufficient charge, or other charge deficiencies.